Most cameras are provided with one or more mechanical systems which, during the sequence of taking a picture, are displaced from one end position to another, and thereafter returned to the first end position in readiness for the taking of a subsequent picture. Such systems include, for example, the viewing mirror mechanism of a single lens reflex camera, and/or the shutter mechanism and/or the diaphragm mechanism.
In terms of the viewing mirror mechanism, a common displacement means includes a return setting member, a mirror-up spring and an associated return spring, a latch and a mirror-up mechanism. A shutter charging operation energizes both the mirror-up spring and the return spring while bringing the return setting member to its set position, where it is locked by the latch. A shutter release causes the mirror-up member to be initially actuated under the influence of the mirror-up spring to drive the mirror upward. Immediately after termination of shutter operation the return setting member is unlocked from the latch, thereby allowing the setting member to be returned under the influence of the return spring until the mirror returns to its original position.
The above-described mechanism suffers from many disadvantages. Firstly, to guarantee that the mirror will indeed reach its end positions, even if the camera in which it is fitted is operated in an inverted state, the two springs must be sufficiently tensioned to overcome mechanical losses so that the mirror arrives at the end positions with at least some tension remaining in the springs. This results in the mirror being accelerated during its entire movement and therefore reaching its end positions at a maximum velocity for that movement. At the end positions the kinetic energy of the mirror and driving mechanism must be absorbed and, due to the relatively high velocity, the deceleration forces can be sufficiently high to cause the camera to shake, thereby giving rise to a blurred picture. The high velocity of the mirror also creates problems with latching of the mirror at its end positions, since the mirror can bounce back before the latch engages. The requisite high deceleration forces lead to increased noise levels and vibration while decreasing the lifespan of the camera.
Many partial solutions to the above-described problem have been proposed. For example, PCT Application No. WO 93/025935 provides a solution for effecting cyclic displacement of a mechanical system, such as a mirror mechanism, shutter mechanism, and the like in a camera, between two end positions, which avoids the risk of bounce, requires a minimal amount of energy and which is simpler than previous solutions. This is achieved by including spring means for biasing the system towards a neutral position or zone between these end positions, and drive means, activated during displacement of the system, for ensuring that the system sequentially reaches these end positions. This arrangement can also be used for cyclic displacement of a mechanical system, such as a mirror mechanism, shutter mechanism, and the like in a camera, between two end positions. The solution consists of the following steps:
I) biasing the system towards a neutral position or zone between the end positions; PA1 II) retaining the system at one of the two end positions; PA1 III) releasing the system from the one end position; PA1 IV) providing an additional force to the system above that due to the biasing to ensure that the system reaches the other of the two end positions; PA1 V) retaining the system at the other end position; PA1 VI) releasing the system from the other end position; and PA1 VII) providing an additional force to the system above that due to the biasing to ensure that the system reaches the one end position.
Since the system is biased towards a neutral position or zone between the end positions, the maximum velocity of the component occurs at this neutral position or zone during movement from one end position to the other. Similarly, the velocity of the system, and therefore its kinetic energy, tends to zero when approaching the end positions. As such, no complicated braking or damping system is necessitated and latching is simplified.
One disadvantage of this solution is its relative lack of speed, i.e. the system cannot react fast enough when a rapid movement is required. This is desired for fast shutter speeds in camera shutters, for example, where high speed movement of the shutter blades or curtains are essential.
Shutter operation detection devices of different types are known. U.S. Pat. No. 5,532,785 describes a system which uses a light emitting diode (LED) and a photo transistor (PTR) placed in proximity to the shutter curtains to measure the actual exposure times. The difference between the control exposure time, which is based upon the brightness value of the subject to be photographed and the film sensitivity, and the measured exposure time, is determined. The control exposure time for the next session can then be corrected based upon this difference. This system measures the time between passing of the front curtain and the rear curtain of a shutter made up of two shutter curtains by detecting reflected light bounced from the LED by means of a curtain blade to the PTR.
One disadvantage with this system is that no information is obtained as to the actual position of the shutter blades at any chosen time in the interval from start of exposure to end of exposure, ie. going from fully closed shutter to fully open shutter and back to fully closed shutter.
It is therefore an object of the present invention to provide an electromagnetic displacement means for effecting cyclic displacement of a mechanical system, such as a shutter mechanism, mirror mechanism, and the like in a camera, between two end positions, which means is capable of high speed movements with repeatability and stability, while requiring a minimal amount of energy and which is simple in construction.